If a tank does need an agro dump for whatever reason, they should be able to call for it and get it. For example, I call for my DPS to agro dump at specific times during Raptus due to the threat debuff from the tank curse. They know where the issues are, and they're happy to oblige when I ask. In turn, I try to provide them with the confidence to blow out their hardest-hitting attacks without fear of turning the boss at an inopportune moment and wiping the raid.
And this is exactly what is required in a progression raiding group. Mutual trust in the group that everyone plays to the best of their ability and reacts to the changing situation. Well put.
Every player in the group is responsible for tanking, dps and healing something might be your main focus but you are still responsible for the others. If tanks and dps take less damage the healers can do more damage. If the tanks hold agro well the dps are more confident to do more damage and in turn avoid more damage because they feel like they don't have to 'push'.
That's why un nerfed DG was the best fight in the game. Everyone was playing all the roles to the best of their ability to down it.